Differential signaling is a method of transmitting information electrically using two complementary signals sent on two separate wires. The signal value is the difference in voltage levels on the two separate wires and this value forms the transmitted signal. The receiving device senses the difference between the two signals and ignores the respective voltages of each wire relative to ground. Therefore, differential signaling enables discrimination of signals at lower levels. This transmission technique can be used for analog signaling and digital signaling. Examples of differential signaling include, but are not limited to, LVDS (Low-Voltage Differential Signaling), differential ECL (Emitter-Coupled Logic), PECL (Positive Emitter-Coupled Logic), LVPECL (Low-Voltage Positive Emitter-Coupled Logic), RS-422, RS-485, Ethernet® (twisted pair), USB (Universal Serial Bus), Serial ATA, TMDS (Transition Minimized Differential Signaling), Firewire® (IEEE 1394), and HDMI (High-Definition Multimedia Interface).
Because differential signaling improves system noise immunity over single-ended signaling, many electronics applications, such as portable or mobile devices, can lower the supply voltage in order to save power and reduce unwanted emitted radiation. For example, the voltages transmitted on the differential lines may be much lower than single-ended signaling. The differential signals level transmitted over TXDP and TXDN is around 200 mv. The difference between TXDP and TXDN represent logic “high” or “low”.
The differential signals may be sent between two devices, a first of which may be a host device (such as a computer, a personal digital assistant, a mobile telephone, etc.) and a second of which may be a device that interfaces with the host device (such as a memory card, WiFi device, or any other peripheral device). One of the devices may send a power-save command, via the differential signals, to another device to enter a power-save state. The power-save state may comprise hibernation or an idle state. The power-save state is a power management mode that conserves power by reducing power to one or more of the components within the device, such as powering down the one or more components within the device. After receiving the power-save command, the device changes its configuration in order to enter the power-save state and in order to exit from the power-save state (when a subsequent command to “wake-up” or exit the power-save state is sent).
One example configuration of the devices is depicted in FIG. 1. On the host/transmitter side, the output transmitter is configured to be in High-Impedance (HIZ). HIZ is the state of an output terminal which is not currently driven by the circuit. In digital circuits, it means that the signal is neither driven to a logic “high” level nor a logic “low” level. Such a signal can be seen as an open circuit (or “floating” wire) because connecting it to a low-impedance circuit will not affect that circuit; it will instead itself be pulled to the same voltage as the actively driven output.
On the card/receiver side, a series of switches are closed/opened in order to configure the receiver. Specifically, low-value resistors (such as 50Ω), used as high-speed termination resistors for ordinary differential signaling, are disconnected from the differential receiver lines (RXDP, RXDN) and high-value resistors (such as a few KΩ) are connected to the differential receiver lines (RXDP, RXDN). The high-value resistors (shown as R-Large in FIG. 1), when combined with the HIZ state of the output transmitter, simulate a logic “low” at the receiver side. The receiver detects the wakeup signal using detector (DET) when the host device exits HIZ and starts driving active signals on RXDP, RXDN.
The configuration in FIG. 1 has several drawbacks. First, the configuration requires two sets of switchable resistors on the receiver side, with one of the sets having a resistance of a few KΩ. Second, the configuration in FIG. 1 is susceptible to electromagnetic interference (EMI) since the values of the R-Large resistors are so high, necessitating more complex operation to reject EMI pulses.
Another example configuration of the devices is depicted in FIG. 2. After the hibernation command is sent, the host sets its transmitter output to be HIZ. And, the host turns on switchable pull up resistors, so that both differential lines are pulled up to V-PULL UP. There is inherent ground shift voltage between host and card. The ground shift voltage is added to the V-PULL UP, and the total level might be overvoltage and destructive to receiver transistors. The configuration depicted in FIG. 2 thus requires the card receiver to include special circuitry to protect the input transistors.
Again, the configuration depicted in FIG. 2 has several drawbacks. First, the host device requires several additional components, including the pull-up resistors. Second, the card receiver requires the additional special circuitry to account for the ground shift. And, the special circuitry limits the frequency range of the differential signaling, only supporting frequencies up to 1.5 Gbps.
Accordingly, solutions for the power-save state suffer from undue complexity.